The invention relates to an optical assembly, more particularly a projection exposure apparatus for microlithography, specifically for EUV lithography, and to a method for operating the optical assembly.
Microlithography projection exposure apparatuses serve for producing microstructured components by means of a photolithographic method. In this case, a structure-bearing mask, the so-called reticle, is imaged onto a photosensitive layer with the aid of a projection optical unit. The minimum feature size that can be imaged with the aid of such a projection optical unit is determined by the wavelength of the imaging light used. The smaller the wavelength of the imaging light used, the smaller the structures that can be imaged with the aid of the projection optical unit. Imaging light having the wavelength of 193 nm or imaging light having a wavelength in the range of the extreme ultraviolet (EUV), i.e. 5 nm-30 nm, is principally used nowadays. With the use of imaging light having a wavelength of 193 nm, both refractive optical elements and reflective optical elements are used within the microlithography projection exposure apparatus. With the use of imaging light having a wavelength in the EUV range, exclusively reflective optical elements (mirrors) are used, which are typically operated under vacuum conditions.
Mirrors of this type have a substrate composed of a material having a low coefficient of thermal expansion, and a reflective coating. If the wavelength of the imaging light used lies in the EUV range between 5 nm and 30 nm, the reflective coating typically comprises individual layers consisting alternately of materials having different refractive indices. During the operation of the projection exposure apparatus, the reflective coating is subjected to EUV radiation, which promotes a chemical reaction of the layer materials used with gaseous substances present in the residual gas atmosphere in the projection exposure apparatus.
In order to protect the individual layers against degradation, a capping layer is typically applied on the reflective coating in this case, which capping layer can consist of ruthenium, for example. However, on such a capping layer, too, a degradation—in the case of ruthenium an oxidation—can occur as a result of a chemical reaction with residual gas present in the vacuum environment, the chemical reaction being initiated or promoted by the EUV radiation. An oxidation of the capping layer during the operation of the projection exposure apparatus leads to a reduction of the reflectivity of the respective mirror and thus to a reduction of the lifetime of said mirror.